1. Field of the Invention
The present invention relates to a titanium alloy with improved machinability and its method of production.
The titanium alloy of the present invention is suitable for a material in the production of, for example, connecting rods to connect automobile engine piston pins and crosshead pins with the crank, or connecting rods for industrial machines.
2. State of the Art
Pure titanium and titanium alloy combine the advantages of light weight and high strength, and are used particularly often as materials in aircraft. These advantages are also applicable in manufacturing automobile or electronic equipment parts and accessories, but because the processability, and especially the machinability, of both Ti and Ti alloys is inferior to that of conventional materials such as steel, the manufacture of parts for mass-produced goods has been difficult.
One way of improving the machinability of steel is to add S. But even if S is added to Ti or Ti alloys, improvements in machinability are not up to the anticipated level, and minuses such as deposition of tabular Ti-S compounds in the grain boundaries and decrease in toughness are conspicuous. For these reasons, up to now a method of improving the machinability, without reducing the properties, of Ti or Ti alloys has not been discovered.
Similar to the addition of Pb to steel to improve its machinability, Pb has been added to titanium alloys in tests to improve their machinability. However, the addition of Pb to titanium alloys is not an effective solution because most of the added Pb dissolves in matrix, and improvements in machinability are no greater than when Pb is added to steel.
For example, connecting rods to connect automobile engine piston pins and crosshead pins with the crank have conventionally used, for the most part, forged parts from iron-based materials. Because the density of iron-based materials is high, there is a limit to how light the connecting rods can be, which becomes an obstacle in the realization of elevation in fuel efficiency with a lightweight engine, or elevation in power through high-speed rotation.
Generally, among the important properties required for connecting rods used in automobiles, industrial machines, and the like, are (1) fatigue strength, (2) toughness, (3) abrasion resistance, and (4) machinability.
Ti alloys possess superior qualities which are able to meet these requirements, and Ti alloy connecting rods are being used for some special purposes (for example, racing cars). A typical alloy is a 6%Al - 4%V - Ti alloy.
However, conventional Ti alloy, starting with the above-mentioned 6%Al - 4%V - Ti composition, has a low thermal conductivity compared to iron-based materials presently being used for connecting rods, and for a high degree of hardness and high activity, workability is extremely poor. Accordingly, industrial production of a large amount of connecting rods using this alloy is difficult, and a particular problem is that the required fatigue strength which is most essential for the connecting rod is insufficient.
Because of this, it was necessary to improve the machinability and fatigue strength of Ti alloys in order to make use of them as connecting rod material in the mass production of automobiles, two-wheeled vehicles, industrial machines, etc.